1. Field of the Invention
The present invention relates to an organic semiconductor transistor element using an organic semiconductor, a semiconductor device using this element, and a process for producing the semiconductor. More specifically, the present invention relates to an organic semiconductor transistor element which can be used suitably in the fields of driving circuits of display elements (such as electronic paper, digital paper, organic EL elements, electrophoresis type display elements or liquid crystal elements), logic circuits or memory elements used in electronic tags, smart cards or the like, gas sensors or the like, semiconductor devices using this element, and also relates to a process for producing the semiconductor device.
2. Description of the Related Art
Thin film transistors are widely used as switching elements for displays, for example, as liquid crystal display elements. To present, thin film transistors have been produced from amorphous or polycrystal silicon. However, the production of thin film transistors from silicon involves production process using a vacuum system such as sputtering or CVD equipment, and involves a very high cost. To produce a thin film transistor, it is necessary to repeat the production process using this kind of vacuum system to form plural layers such as a semiconductor layer; therefore, in order to make a large-sized display device or other device using a thin film transistor, there is also a large increase in production cost.
The process for forming a film from amorphous or polycrystal silicon is performed at a very high temperature. Thus, material which can be used as a substrate for the film is limited. Consequently, the problem remains of an inability to use a light and flexible substrate, such as a resin substrate.
Organic semiconductors, a typical example of which is an organic EL element, have also undergone active research in recent years. In tandem with this, there have also been studies of integrating organic material into a circuit, the organic material having characteristics of lightness and flexibility that silicon material does not have.
A low molecular weight compound or a polymer compound is adopted for the organic material used in this kind of thin film transistor. For the low molecular weight compound, polyacene compounds such as pentacene and tetracene (see, for example, Japanese Patent Application Laid-Open (JP-A-) Nos. 5-55568, 10-270712 and 2001-94107) and phthalocyanine compounds such as copper phthalocyanine are suggested (see, for example, JP-A Nos. 5-190877 and 2000-174277).
However, in the case of the low molecular weight compound, it is necessary to repeat the production process using the same vacuum system as used in the case of silicon. Thus, the previously mentioned problems relating to the production process also exist in this case.
For the polymer compound, aromatic oligomers such as sexithiophene (see, for example, JP-A No. 8-264805), and polymer compounds such as polythiophene, polythienylvinylene, and poly(p-phenylenevinylene) are suggested (see, for example, JP-A Nos. 8-228034, 8-228035, 10-125924 and 10-190001, and Appl. Phys. Lett., vol. 73, 108 (1998)).
This kind of polymer compound is advantageous in the production process since the compound has a greater solubility and can be formed into a film through a low-cost technique such as spin coating or dip coating. However, there exists a problem with the polymer compound that the mobility of carriers contained therein is low. In the case of poly(p-phenylenevinylene), since a soluble precursor is subjected to spin coating followed by heat treatment, defects are easily formed in the main chain conjugated system of the polymer so as to produce a marked lowering in the electrical characteristic thereof.